“The AO index describes the
relative intensity of a semipermanent low-pressure center over the North
Pole. A band of upper-level winds circulates around this center, forming a
vortex. When the AO index is positive and the vortex intense, the winds
tighten like a noose around the North Pole, locking cold air in place. A
negative AO and weak vortex … allow intrusions of cold air to plunge
southward into North America, Europe, and Asia. … the index has been mostly
positive in wintertime since the late 1980s. The Arctic Oscillation has
strengthened in recent decades, contributing to the unusual warmth over the
Northern Hemisphere land masses.” [http://www.ucar.edu/communications/newsreleases/2003/deser.html]

The following figure provides another view of the same
data – “the standardized
seasonal mean AO index during cold season (blue line) is constructed by
averaging the daily AO index for January, February and March for each year.
The black line denotes the standardized five-year running mean of the index.
Both curves are standardized using 1950-2000 base period statistics.”
[http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/JFM_season_ao_index.shtml]

The National Snow and Ice Data
Center states: “Over most of the past century, the Arctic Oscillation
alternated between its positive and negative phases. Starting in the 1970s,
however, the oscillation has tended to stay in the positive phase, causing
lower than normal arctic air pressure and higher than normal temperatures in
much of the United States and northern Eurasia.”
[http://nsidc.org/arcticmet/patterns/arctic_oscillation.html]

The following figures show: Left: winter
(DJF) sea-level pressure (SLP) averaged over the period 1900-2001. The AO
index refers to opposing atmospheric pressure patterns in northern middle and
high latitudes. The blue areas are low pressure areas. Right: modern
distribution of permafrost in the Northern Hemisphere. Continuous permafrost
is shown by dark blue colour. Discontinuous and sporadic permafrost is shown
by light blue color. Red and black arrows show main surface air flow (warm
and cold, respectively) as generated by the 20th century pattern
of SLP. The overall wind systems set up by the average winter sea-level
pressure appears to represent one of several controls on the present
distribution of permafrost in the northern hemisphere. [http://www.climate4you.com/NAOandAO.htm]

An article in Science Daily: “Winds, Ice Motion Root Cause
Of Decline In Sea Ice, Not Warmer Temperatures” [http://www.sciencedaily.com/releases/2004/12/041220010410.htm]
states: “Extreme
changes in the Arctic Oscillation in the early 1990s -- and not warmer
temperatures of recent years -- are largely responsible for declines in how
much sea ice covers the Arctic Ocean, with near record lows having been
observed during the last three years, University of Washington researchers
say. It may have happened more than a decade ago, but the sea ice appears to
still "remember" those Arctic Oscillation conditions, according to
Ignatius Rigor, a mathematician with the UW's Applied Physics Laboratory”

A 2010 paper (Ohashu and
Tanaka: “Data Analysis of Recent Warming Pattern in the Arctic”, SOLA 2010
Vol 6A [http://www.jstage.jst.go.jp/article/sola/6A/SpecialEdition/1/_pdf]
) states: “variability of
the global atmosphere, it is shown that both of decadal variabilities before
and after 1989 in the Arctic can be mostly explained by the natural
variability of the AO not by the external response due to the human activity”.

The following figure superimposes the Saentis temperatures
from above on the AO shown previously. The Alps regime shift observed in the
1988 time frame is clearly correlated with the abrupt change in the AO.

The AO had a regime shift in 1988/89, examined in
detail along with the AO-solar connections.

The North Atlantic Oscillation (NAO) is a major mode of
atmospheric variability in the Northern Hemisphere, particularly in winter.
The NAO index is calculated based on the difference between the normalized
sea level pressures over Gibraltar (or Portugal, or the Azores) (subtropical
high) and Southwest Iceland (polar low).

More and stronger winter storms crossing the Atlantic
Ocean on a more northerly track

Fewer and weaker winter storms crossing on a more
west-east pathway

Warm and wet winters in Europe

Moist air into the Mediterranean and cold air to
northern Europe

Cold and dry winters in northern Canada and Greenland

Milder winter temperatures in Greenland

US east coast experiences mild and wet winter conditions

US east coast experiences more cold air outbreaks and
hence snowy weather conditions

The following figures show the general patterns of the NAO
positive and negative modes (from NOAA Airmap [http://airmap.unh.edu/background/nao.html],
which provides a comprehensive explanation of NAO effects on the eastern US).

Studies of
NAO Effects

An article in Science Daily (“North Atlantic Warming Tied
To Natural Variability”, Jan 2008 [http://www.sciencedaily.com/releases/2008/01/080103144416.htm])
reported on a Duke University study of North Atlantic temperatures and their
relation to the NAO. “while the North Atlantic Ocean's surface waters warmed in the 50
years between 1950 and 2000, the change was not uniform. In fact, the
subpolar regions cooled at the same time that subtropical and tropical waters
warmed. "It is premature to conclusively attribute these regional
patterns of heat gain to greenhouse warming," … water in the
sub-polar ocean --- roughly between 45 degrees North latitude and the Arctic
Circle --- became cooler as the water directly exchanged heat with the air
above it. …NOA[sic] -driven winds served to "pile up" sun-warmed
waters in parts of the subtropical and tropical North Atlantic south of 45
degrees” … "We suggest that the large-scale, decadal
changes...associated with the NAO are primarily responsible for the ocean
heat content changes in the North Atlantic over the past 50 years,"
the authors concluded.”

Studies have found correlations between NAO and Indian
monsoon. The study “Interannual and long-term variability in the North
Atlantic Oscillation and Indian summer monsoon rainfall” (Dugam, Kakade and
Verma, Indian Institue of Tropical Meteorology, 1997 [http://cat.inist.fr/?aModele=afficheN&cpsidt=2068184])
investigated 108 years of NAO / monsoon data and found that: “The decadal scale analysis
reveals that the NAO during winter (December-January-February) and spring
(March-April-May) has a statistically significant inverse relationship with
the summer monsoon rainfall of Northwest India, Peninsular India and the
whole of India. The highest correlation is observed with the winter NAO.
The NAO and Northwest India rainfall relationship is stronger than that for
the Peninsular and whole of India rainfall on climatological and
sub-climatological scales.”

A study of the multi-decadal low frequency oscillation
(LFO) of the NAO [http://denali.frontier.iarc.uaf.edu:8080/~igor/research/pdf/50yr_web.pdf]
stated: “observations
over the past 135 years showed that the recent decrease in ice extent in
the Nordic Seas is within the range of natural variability since the 18th
century. A combination of century- and half-a-century-long data records
and model integrations leads us to conclude that the natural low-frequency
oscillation (LFO) exists and is an important contributor to the recent
anomalous environmental conditions in the Arctic. There is evidence that the
LFO has a strong impact on ice and ocean variability”.

A NASA JPL study (NASA Finds Sun-Climate Connection in Old
Nile Records March, 2007 [http://www.jpl.nasa.gov/news/features.cfm?feature=1319])
stated: “Direct
measurements of light energy emitted by the sun, taken by satellites and
other modern scientific techniques, suggest variations in the sun's activity
influence Earth's long-term climate. However, there were no measured climate
records of this type until the relatively recent scientific past. The authors
suggest that variations in the sun's ultraviolet energy cause adjustments in
a climate pattern called the Northern Annular Mode, which affects climate in
the atmosphere of the Northern Hemisphere during the winter. At sea level,
this mode becomes the North Atlantic Oscillation, a large-scale seesaw in atmospheric
mass that affects how air circulates over the Atlantic Ocean. During
periods of high solar activity, the North Atlantic Oscillation's influence
extends to the Indian Ocean.”

A study of the NAO climate effects on barn swallow
breeding (“North Atlantic Oscillation (NAO)
effects of climate on the relative importance of first and second clutches in
a migratory passerine bird”) http://www3.interscience.wiley.com/journal/118942055/abstract?CRETRY=1&SRETRY=0
“The size of first
clutches increased with increasing NAO values, and clutch size increased
during the study period. [1970-2000] … in years with high NAO index values
first broods were relatively larger than second broods compared to years with
low NAO values. Similarly, the breeding success of first relative to second
broods was larger in years with high NAO index values compared to years with
low NAO values”

A study of “Midlatitude Forcing Mechanisms for Glacier Mass
Balance” (Reichert, Bengtsson and Oerlemans, Journal of Climate, 2000) [http://www.mpimet.mpg.de/fileadmin/publikationen/Ex72.pdf]
states: “The observed
strong positive glacier mass balances within 1980-1995 can thus be seen as a
consequence of the corresponding observed persistent positive phase of the
NAO within this time period. This mechanism will probably also be valid for
other maritime Norwegian glaciers which have also shown strong positive mass
balances within this period [IASH, 1999].”

The following figure shows the correlation between the NAO
index (black) and the Nigardsbreen Glacier (Norway) mass balance (green).

The same study concludes: “Glacier mass balances of Alpine
glaciers have been strongly negative during the decade 1980-1990. The average
mass balance value has been -0.65 mwe [Haeberli et al., 1999]
resulting in an estimated loss of 10 to 20% of ice volume since 1980 [Haeberli
and Beniston, 1998]. The results of the present study indicate that this
loss of mass balance can (at least as has been demonstrated for
Rhonegletscher) partly be explained by the strong positive phase of the NAO
within this time period.”

When the polar vortex weakens, the polar jet stream slows
and meanders in a form that allows the extension of low pressure lobes much
farther to the south. These can become stationary for days and block the
normal circulation of the atmosphere. The negative
AO/NAO is associated with a slowed polar vortex and polar jet stream. When
the jet stream slows, it meanders in a waveform pattern (Rossby waves). The
general effect is illustrated below (fromhttp://en.wikipedia.org/wiki/Rossby_wave).

“Blocking has a lot to do with
the severity of winter weather in the Northern Hemisphere, and the AO has a
strong ability to control blocking. To quantify this control, we define a
blocking event as a week or more of excess pressure in the midtroposphere together
with an anticyclone at the surface. … blocking occurs preferentially during
the low AO phase in Alaska, the North Atlantic, and Russia.” [http://www.jisao.washington.edu/wallace/ncar_notes/]

Barriopedro et al (“Solar
modulation of Northern Hemisphere winter blocking”, Journal of
Geophysical Research, Vol.113, 2008 [http://idl.ul.pt/davidbarriopedro/2008%20Barriopedro%20et%20al.pdf])
found that “Solar
activity modulates the preferred locations for blocking occurrence over both Oceans,
causing local frequency responses therein. Over the Pacific Ocean high/low solar
activity induces an enhanced blocking activity over its eastern/western part.
Atlantic blocking occurrence increases for both (high/low) solar phases, with
a spatial dependent response confined to western/eastern Atlantic. … Low
solar Atlantic blocking episodes last longer, are located further east and
become more intense than high solar blocking events.”